Why is iron the threshold beyond which heavy-nucleus FISSION releases energy, but nuclei lighter than iron release energy via FUSION? (Use the binding energy per nucleon curve to answer.)
AIron has the HIGHEST binding energy per nucleon; reactions move nuclei toward it
BIron has the lowest binding energy per nucleon, so all reactions go through it
CIron is the most common element on Earth, so it sets the scale
DIron has a unique magnetic moment that aligns nuclear spins
Answer & Solution
Correct answer: A. Iron has the HIGHEST binding energy per nucleon; reactions move nuclei toward it
1. Energy is released in a nuclear reaction whenever the PRODUCTS have a higher TOTAL binding energy than the REACTANTS.
2. The BE/nucleon curve peaks at $A \approx 56$ ($\,^{56}\mathrm{Fe}$) at $\sim 8.8\,\text{MeV}/\text{nucleon}$.
3. LIGHT nuclei (e.g. H, D, He) are LEFT of the peak — they have low BE/nucleon. FUSING them into a heavier nucleus moves products CLOSER to iron, INCREASING average BE/nucleon → energy released.
4. HEAVY nuclei (e.g. uranium, plutonium) are RIGHT of the peak. FISSIONING them into two mid-mass fragments ALSO moves products closer to iron → BE/nucleon increases → energy released.
5. Therefore iron is the "valley of stability": every spontaneous reaction tends to push nuclei toward $\,^{56}\mathrm{Fe}$, the most tightly bound region.
6. Option A states the OPPOSITE (iron has the HIGHEST, not lowest, BE/nucleon). Options C and D are unrelated misconceptions.
_Source: NCERT Class 12 Physics Part 2, Ch 13, §13.7 (Nuclear Energy — fission and fusion both arise from the BE/A curve), p. 9–11._
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